Power semiconductor devices IGBTs, MOSFETs and diodes play the role of switching electric power at speeds of hundreds of kHz in power electronic conversion systems such as motor drives. The core element of these power switches is one or more semiconductor die (note the term die is used in both the singular and plural sense) each with a dimension of, for example 10 mm×10 mm×0.1 mm with rated current/voltage at hundreds of amperes (A) and hundreds of volts (V).
To facilitate high power conversion circuit configurations such as half- or full-bridge, three-phase, matrix, etc., power modules are built up by interconnecting multiple die and then encapsulating them into one housing or package. These processes are typically referred to as power module packaging. During operation, the die dissipate large amounts of heat, on the order of hundreds watts (W) per die, leading to high operating temperatures.
Semiconductor devices and associated packaging materials present inherent temperature limitations. For example, the maximum operating temperature of a silicon (Si) device is about 150° C. Furthermore, high temperature and large temperature excursion during their operation seriously affect the lifetime and/or reliability of power modules. Consequently, cooling design strategies including strict requirements for a power module's thermal performance are necessary to control device (die) temperatures. In addition, another main function of a power module structure is to supply mechanical support in harsh environments including high stress environments including high stress vibration conditions.
The characterization of power module performance includes characterization of specific operating parameters, for example, thermal resistance, parasitic inductance, power density, and number to failure, etch, all of which are related to the physical structure and material properties of the power module package.
For example, a conventional wire-bonded power module package in the prior art, as shown in FIG. 1A, employs bonding wire and solder to respectively connect the electrodes on the top and bottom of the die, e.g., 14, onto the etched circuit in a direct bond copper (DBC) substrate, e.g., including a ceramic plate 12B sandwiched between double side Cu layers 12A and 12C. The power and signal input/output (I/O) is carried out by soldering Cu terminals (e.g., 16A and 16C) on the DBC substrate. This assembly is then soldered onto a flat Cu baseplate 12 and encapsulated in a plastic housing (e.g., 18) filled with a polymer protective (filling) to perform mechanical support and electrical isolation. The module with baseplate 12 is then bolted with bolts e.g., 24 onto a heatsink 22 with thermal paste (or grease) between the baseplate 12 and heatsink 22 to reduce the contact thermal resistance. A power busbar 26 is attached to power terminals on the housing (e.g., copper terminals 16A) as well as including signal terminals e.g., 16C for next level electrical connection.
It is known that the thermal resistivity (thermal resistance per unit area) includes thermal resistance from different layers in series with an original heat source (die) to the outside coolant in contact with the heatsink. The various layers include the top solder layer, DBC layers (Cu-Ceramic-Cu), bottom solder layer, baseplate, thermal paste and heat sink/cooler wall (half) to the coolant.
FIG. 1B shows the thermal resistivity of the various layers of the power module package where the thermal grease, solder, baseplate and heat sink wall contribute the greatest proportion to the total thermal resistance of the power module assembly.
There have been several approaches in the prior art to provide a low thermal resistance power module assembly in contact with a coolant. For example Hassani et al. disclose a power module where heat is dissipated through an abutting circuit substrate, which may be a direct bonded copper (DBC) substrate, to a spreader plate which includes a coolant passageway or opening that allows the coolant to directly contact the DBC substrate.
Mizzi (U.S. Pat. No. 5,245,508) discloses a cooling method for circuit boards where a plurality of heat conductive flexible membranes containing coolant fluids are compressed between adjacent circuit boards to conform to the surface of the individual circuit boards to dissipate heat.
Lamers (U.S. Pat. No. 5,262,921) discloses a method for cooling circuit boards where the non-circuit bearing side is exposed directly to a coolant stream.
Anderson (U.S. Pat. No. 5,274,530) discloses a module for cooling semiconductor die where two printed circuit wiring boards with a plurality of semiconductor die mounted thereon form two opposite inside surfaces of a module and are attached to outer cover plates. The module is received in a chassis with slots which are formed by the sidewalls in the chassis. Coolant passes between the sidewalls to remove heat from the module.
Miller et al. (U.S. Pat. No. 6,400,012) discloses a heat sink for cooling an integrated circuit where a semiconductor device is mechanically and electrically coupled to the top surface of a dielectric substrate having one or more wiring layers therein. A fluid flow channel defined in a back-side surface of the substrate for passing a cooling medium.
Prior art cooling methods and modules have several shortcomings. For example, due to complex cooling configuration, such prior art modules have serious limitations including undesirably high thermal resistance, poor thermo-mechanical reliability, undesirably high weight, and undesirably high cost of manufacture, as well as producing undesired electrical parasitic effects caused by lack of special electrical considerations.
Thus, there is a need for improved power module structures and packages in the power module packaging art to overcome problems in the prior art including providing for improved heat dissipation, reduced parasitic effects, reduced volume, reduced weight, and improved reliability in harsh environments, all of the foregoing achieved at a reduced cost.
Therefore it is an object of the invention to provide an improved power module structure and package to provide improved heat dissipation, reduced parasitic effects, reduced volume, reduced weight, and improved reliability in harsh environments, all of the foregoing achieved at a reduced cost.